This invention relates generally to load responsive fluid control valves and to fluid power systems incorporating such valves, which systems are supplied by a single fixed or variable displacement pump. Such control valves can be used in a multiple load system, in which a plurality of loads are individually controlled under positive and negative load conditions by separate control valves.
In more particular aspects this invention relates to direction and flow control valves capable of controlling simultaneously multiple positive and negative loads, which while controlling a negative load interrupt pump flow to the motor, providing the motor inlet with fluid from the pressurized system exhaust.
Closed center load responsive fluid control valves are very desirable for a number of reasons. They permit load control with reduced power losses and therefore, increased system efficiency and when controlling one load at a time provide a feature of flow control irrespective of the variation in the magnitude of the load. Normally such valves include a load responsive control, which automatically maintains pump discharge pressure at a level higher, by a constant pressure differential, than the pressure required to sustain the load. A variable orifice, introduced between pump and load, varies the flow supplied to the load, each orifice area corresponding to a different flow level, which is maintained constant irrespective of the variation in magnitude of the load. A fluid control valve for such a system is shown in U.S. Pat. No. 3,488,953 issued to Haussler. The application of such a system is, however, limited by one basic system disadvantage.
Normally in such a system the load responsive valve uses flow from the pump not only while maintaining a constant pressure differential and therefore constant flow characteristics when operating a positive load, but also uses pump flow during operation of negative load.
This drawback can be overcome in part by provision of fluid control valves as disclosed in U.S. Pat. No. 3,807,447 issued to Masuda on Apr. 30, 1974. However, while these valves utilize actuator exhaust fluid for actuator inlet flow requirement when controlling negative loads, they regulate actuator inlet pressure by bypassing fluid to a down stream load circuit. Masuda's valves and their proportional control system are based on series type circuit in which excess fluid flow is successively diverted from one valve to the other and in which loads arranged in series determine the system pressure. In such a system flow to the last valve operating a load must be delivered through all of the bypass sections of all of the other system valves, resulting in fluid throttling loss. These valves are not adaptable to simultaneous control of multiple loads in parallel circuit and they do not provide system load control pressure signal to the pump flow control mechanism. The controllers of these valves respond to pressure differential developed across a metering orifice and not to negative load pressure.